Elimination Reactions of Hydrazonium Salts: Experimental and Theoretical Evidence for a Large Stereoelectronic Effect of Nitrogen

Article abstract of DOI:10.1021/jo00312a026

Elimination of trimethylamine from hydrazonium salts (8, R = Me) is promoted by base (methoxide ion in methanol).The mechanism is characterized as E2 as shown by substituent effects (ρ = +2.57), Broensted coefficients, a primary kinetic isotope effect (k_H/k_D = 2.10), and solvent effects.Variation in the leaving group (8, R = arylmethyl) shows that N-N bond cleavage is less well advanced in the transition state than C-H bond breaking.The elimination to give the nitrile is syn-periplanar and, using the phthalazinium salt 4 as a model for the anti elimination, an anti/syn ratio of ca. 10² is found.The reactions have been modelled using ab initio methods with the transition structures located at the HF/3-21G level and relative energies computed using the MP2/6-31G* method.Using both H2O and NH3 as model bases to induce elimination on ⁺, the calculated energy for anti elimination is lower (in the range 11-13 kcal mol^-1) than that for syn elimination.The implications of the results for the ease of formation of nitriles from aldehydes via the hydrazonium salt route are discussed.